a. Field of the Invention
This invention relates to catheters for diagnosing and treating tissue, particularly human cardiac tissue. In particular, the invention relates to an ablation catheter with a collapsible tip that contains a virtual electrode interposed between two sensor arrays at a distal portion of the catheter.
b. Background Art
Catheters have been in use for medical procedures for many years. Catheters can be used for medical procedures to examine, diagnose, and treat while positioned at a specific location within the body that is otherwise inaccessible without more invasive procedures. During these procedures a catheter is inserted into a vessel near the surface of the body and is guided to a specific location within the body for examination, diagnosis, and treatment. For example, one procedure utilizes a catheter to convey an electrical stimulus to a selected location within the human body. Another procedure utilizes a catheter with sensing electrodes to monitor various forms of electrical activity in the human body.
Catheters are also used increasingly for medical procedures involving the human heart. Typically, the catheter is inserted in an artery or vein in the leg, neck, or arm of the patient and threaded, sometimes with the aid of a guide wire or introducer, through the vessels until a distal tip of the catheter reaches the desired location for the medical procedure in the heart.
A typical human heart includes a right ventricle, a right atrium, a left ventricle, and a left atrium. The right atrium is in fluid communication with the superior vena cava and the inferior vena cava. The atrioventricular septum separates the right atrium from the right ventricle. The tricuspid valve contained within the atrioventricular septum provides communication between the right atrium and the right ventricle.
In the normal heart, contraction and relaxation of the heart muscle (myocardium) takes place in an organized fashion as electro-chemical signals pass sequentially through the myocardium from the sinoatrial (SA) node, which comprises a bundle of unique cells disposed in the wall of the right atrium, to the atrioventricular (AV) node and then along a well-defined route, which includes the His-Purkinje system, into the left and right ventricles. The AV node lies near the ostium of the coronary sinus in the interatrial septum in the right atrium. Each cell membrane of the SA node has a characteristic tendency to leak sodium ions gradually over time such that the cell membrane periodically breaks down and allows an inflow of sodium ions, thereby causing the SA node cells to depolarize. The SA node cells are in communication with the surrounding atrial muscle cells such that the depolarization of the SA node cells causes the adjacent atrial muscle cells to depolarize. This results in atrial systole, wherein the atria contract to empty and fill blood into the ventricles. The atrial depolarization from the SA node is detected by the AV node which, in turn, communicates the depolarization impulse into the ventricles via the bundle of His and Purkinje fibers following a brief conduction delay. The His-Purkinje system begins at the AV node and follows along the membranous interatrial septum toward the tricuspid valve through the atrioventricular septum and into the membranous interventricular septum. At about the middle of the interventricular septum, the His-Purkinje system splits into right and left branches which straddle the summit of the muscular part of the interventricular septum.
Sometimes abnormal rhythms occur in the heart, which are referred to generally as arrhythmia. For example, a common arrhythmia is Wolff-Parkinson-White syndrome (W-P-W). The cause of W-P-W is generally believed to be the existence of an anomalous conduction pathway or pathways that connect the atrial muscle tissue directly to the ventricular muscle tissue, thus bypassing the normal His-Purkinje system. These pathways are usually located in the fibrous tissue that connects the atrium and the ventricle.
Other abnormal arrhythmias sometimes occur in the atria, which are referred to as atrial arrhythmia. Three of the most common atrial arrhythmia are ectopic atrial tachycardia, atrial fibrillation, and atrial flutter. Atrial fibrillation can result in significant patient discomfort and even death because of a number of associated problems, including the following: an irregular heart rate, which causes patient discomfort and anxiety; loss of synchronous atrioventricular contractions, which compromises cardiac hemodynamics, resulting in varying levels of congestive heart failure; and stasis of blood flow, which increases the likelihood of thromboembolism.
Efforts to alleviate these problems in the past have included significant usage of pharmacological treatments. While pharmacological treatments are sometimes effective, in some circumstances drug therapy has had only limited effectiveness and is frequently plagued with side effects, such as dizziness, nausea, vision problems, and other difficulties.
An increasingly common medical procedure for the treatment of certain types of cardiac arrhythmia is catheter ablation. During conventional catheter ablation procedures, an energy source is placed in contact with cardiac tissue to heat the tissue and create a permanent scar or lesion that is electrically inactive or noncontractile. During one procedure, the lesions are designed to interrupt existing conduction pathways commonly associated with arrhythmias within the heart. The particular area for ablation depends on the type of underlying arrhythmia. One common ablation procedure treats atrioventricular nodal reentrant tachycardia (AVNRT). Ablation of fast or slow AV nodal pathways is disclosed in Singer, I., et al., “Catheter Ablation for Arrhythmias,” Clinical Manual of Electrophysiology, pgs. 421–431 (1993).
Another medical procedure using ablation catheters with sheaths to ablate accessory pathways associated with W-P-W utilizing both a transseptal and retrograde approach is discussed in Saul, J. P., et al., “Catheter Ablation of Accessory Atrioventricular Pathways in Young Patients: Use of long vascular sheaths, the transseptal approach and a retrograde left posterior parallel approach,” Journal of the American College of Cardiology, Vol. 21, no. 3, pgs. 571–583 (1 Mar. 1993). Other catheter ablation procedures are disclosed in Swartz, J. F., “Radiofrequency Endocardial Catheter Ablation of Accessory Atrioventricular Pathway Atrial Insertion Sites,” Circulation, Vol. 87, no. 2, pgs. 487–499 (February 1993).
Ablation of a specific location within or near the heart requires the precise placement of the ablation catheter. Precise positioning of the ablation catheter is especially difficult because of the physiology of the heart, particularly because the heart continues to beat throughout the ablation procedures. Commonly, the choice of placement of the catheter is determined by a combination of electrophysiological guidance and fluoroscopy (placement of the catheter in relation to known features of the heart, which are marked by radiopaque diagnostic catheters that are placed in or at known anatomical structures, such as the coronary sinus, high right atrium, and the right ventricle).
The energy necessary to ablate cardiac tissue and create a permanent lesion can be provided from a number of different sources. Originally, direct current was utilized although laser, microwave, ultrasound, and other forms of energy have also been utilized to perform ablation procedures. Because of problems associated with the use of direct current, however, radiofrequency (RF) has become the preferred source of energy for ablation procedures. The use of RF energy with an ablation catheter contained within a transseptal sheath for the treatment of W-P-W in the left atrium is disclosed in Swartz, J. F. et al., “Radiofrequency Endocardial Catheter Ablation of Accessory Atrioventricular Pathway Atrial Insertion Sites,” Circulation, Vol. 87, pgs. 487–499 (1993). See also Tracey, C. N., “Radio Frequency Catheter Ablation of Ectopic Atrial Tachycardia Using Paced Activation Sequence Mapping,” J. Am. Coll. Cardiol. Vol. 21, pgs. 910–917 (1993).
In addition to radiofrequency ablation catheters, thermal ablation catheters have been used. During thermal ablation procedures, a heating element, secured to the distal end of a catheter, heats thermally conductive fluid, which fluid then contacts the human tissue to raise its temperature for a sufficient period of time to ablate the tissue.
Conventional ablation procedures utilize a single distal electrode secured to the tip of an ablation catheter. Increasingly, however, cardiac ablation procedures utilize multiple electrodes affixed to the catheter body. These ablation catheters often contain a distal tip electrode and a plurality of ring electrodes.
To form linear lesions within the heart using a conventional ablation tip electrode requires the utilization of procedures such as a “drag burn.” The term “linear lesion” as used herein means an elongate, continuous lesion, whether straight or curved, that blocks electrical conduction. During a “drag burn” procedure, while ablating energy is supplied to the tip electrode, the tip electrode is drawn across the tissue to be ablated, producing a line of ablation. Alternatively, a series of points of ablation are formed in a line created by moving the tip electrode incremental distances across the cardiac tissue. The effectiveness of these procedures depends on a number of variables including the position and contact pressure of the tip electrode of the ablation catheter against the cardiac tissue, the time that the tip electrode of the ablation catheter is placed against the tissue, the amount of coagulum that is generated as a result of heat generated during the ablation procedure, and other variables associated with a beating heart, especially an erratically beating heart. Unless an uninterrupted track of cardiac tissue is ablated, unablated tissue or incompletely ablated tissue may remain electrically active, permitting the continuation of the stray circuit that causes the arrhythmia.
It has been discovered that more efficient ablation may be achieved if a linear lesion of cardiac tissue is formed during a single ablation procedure. The ablation catheters commonly used to perform these ablation procedures produce electrically inactive or noncontractile tissue at a selected location by physical contact of the cardiac tissue with an electrode of the ablation catheter. Conventional tip electrodes with adjacent ring electrodes cannot perform this type of procedure, however, because of the high amount of energy that is necessary to ablate sufficient tissue to produce a complete linear lesion. Also, conventional ring electrode ablation may leave holes or gaps in a lesion, which can provide a pathway along which unwanted electrochemical signals can travel.
During conventional ablation procedures, the ablating energy is delivered directly to the cardiac tissue by an electrode on the catheter placed against the surface of the tissue to raise the temperature of the tissue to be ablated. This rise in tissue temperature also causes a rise in the temperature of blood surrounding the electrode, which often results in the formation of coagulum on the electrode, which reduces the efficiency of the ablation electrode. With direct contact between the electrode and the blood, some of the energy targeted for the tissue ablation is dissipated into the blood. To achieve efficient and effective ablation, coagulation of blood that is common with conventional ablation catheters should be avoided. This coagulation problem can be especially significant when linear ablation lesions or tracks are produced because such linear ablation procedures conventionally take more time than ablation procedures ablating only a single location.
As noted, before treatment by ablation, clinicians attempt to determine the actual path of travel of the stray electrical signals through cardiac tissue that cause the arrhythmia. Once an ablation procedure has been performed, it is necessary to determine whether the lesion formed is effective in circumventing the unwanted electrical pathways. At present this involves the alternate or concurrent use of sensing catheters. This means that either the ablation catheter is removed from the cardiac cavity and a sensing catheter is inserted in its place to measure the efficacy of the procedure, or additional sensing catheters are introduced simultaneously into the cardiac cavity. Both of these options are problematic. If the ablation catheter is removed and replaced with a sensing catheter, it is difficult to reposition the ablation catheter in the original position should additional energy need to be applied to increase the size or depth of the lesion to arrest the signals if the first ablation attempt was unsuccessful. In the alternate method, the introduction of additional sensing catheters simultaneously with the ablation catheter means that additional incisions must be made in the patient and additional vessels used to navigate the catheters into the heart. This increases the risk of the procedure to the patient and also causes greater patient discomfort.
In some instances, stray electrical signals find a pathway down the pulmonary veins and into the left atrium of the heart. In these instances, it may be advantageous to produce a circumferential lesion at or near the ostium of one or more of the pulmonary veins. Desirably, such a circumferential lesion would electrically isolate a pulmonary vein from the left atrium, completely blocking stray signals from traveling down the pulmonary vein and into the left atrium. It is desirable to have a catheter with a distal portion for forming such circumferential lesions in tissue while avoiding problems with existing designs.
The information included in this background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.